I don't think anyone will really know the answer, because usually people will buy either one but not both, which are pretty much the same products in the same market.
Also there are many models of M400 and STAG, there are the comp guns with SS barrel and the regular barrels. So it must be orange to orange, apple to apple.
If they are rack grade rifles, they really should be about the same. I have a SIG 516 and it shoots 2.5MOA with a reflex sight at 200m with XM855 monopoling, which is the better end of the norm for this kind of rifle using this sort of ammo with an aimpoint. I don't think the STAG or M400 will perform any drastically worse or better.